This invention relates to data communication between presentation layers which are defined according to open systems interconnection (OSI) standardized by the International Organization for Standardization (ISO).
A first or lowest layer through a seventh or highest layer are defined by the International Standard ISO 7498 "Information Processing Systems--Open Systems Interconnection--Basic Reference Model". The presentation layer is the sixth layer, on which an application layer lies as the seventh layer.
In the manner which will later be described more in detail, a presentation protocol machine (PPM) is a data communication system operable to function as the presentation layer and may be called a first data communication system. The data communication system comprises an application program unit for producing and accepting data represented by abstract syntaxes and a sending or transmitting and receiving unit for sending and receiving data through a communication line of an open systems interconnection network including at least one counterpart presentation protocol machine that may be called a second data communication system. In the communication line, the data are represented by transfer syntaxes. In the presentation protocol machine, the abstract syntaxes are encoded into the transfer syntaxes for sending to the communication line. When received, the transfer syntaxes are decoded into the abstract syntaxes.
Before supply to the communication line, the data are represented as the abstract syntaxes according to the International Standard ISO 8824 "Information Processing Systems--Open Systems Interconnection--Specification of Abstract Syntax Notation One (ASN. 1)". In general, each abstract syntax is defined in connection with an individual function, such as an abstract syntax for use in file transfer, and is given an abstract syntax name. Such abstract syntax names are administered by an international organization for registration or a registration authority (RA) so as to be unequivocal.
Each data communication system is preliminarily informed of either all the abstract syntax names or at least a part of the abstract syntax names. Before the first data communication system transmits or receives the data to or from the second data communication system, negotiation is had between operators of the first and the second data communication systems as regards the abstract syntaxes to decide a set of abstract syntaxes which should be used in data transfer.
After the negotiation, the first data communication system encodes the abstract syntaxes into the transfer syntaxes and transmits the transfer syntaxes towards the second data communication system with information attached before each transfer syntax to specify its abstract syntax name. After the negotiation, the first data communication system receives like transfer syntaxes from the second data communication system together with similar information preceding each transfer syntax and decodes the transfer syntaxes into the abstract syntaxes according to the similar information.
Inasmuch as the data transfer is carried out in this manner between at least two conventional data communication systems, the negotiation has been indispensable before the data transfer between the first and the second data communication systems to decide abstract syntaxes of a set for use in the data transfer. In other words, the first data communication system has been unable to transmit the data to the second data communication system unless the data can be described as abstract syntaxes of at least one definition that the second data communication system is aware of prior to the data transfer.
From a different point of view, a method out communication between at least two presentation layers defined by open systems interconnection standardized by the International Organization for Standardization and is for use in each of the presentation layers. A conventional method of the type described, include the steps of producing abstract syntaxes descriptive of data to be transmitted, negotiating between the above-mentioned each of the presentation layers and a different one of the presentation layers as regards the abstract syntaxes to decide a set of abstract syntaxes as negotiated syntaxes, encoding the negotiated syntaxes into transfer syntaxes with the transfer syntaxes accompanied by information indicative of the negotiated syntaxes, and sending or transmitting the transfer syntaxes to the different presentation layer together with the information.
In this manner, the conventional data communication method can deal with the negotiated syntaxes, which are preliminarily negotiated ones of the abstract syntaxes. The conventional method is, however, incapable of dealing with other abstract syntaxes, for which the negotiation is not preliminarily completed